In medical diagnostic imaging X-radiation is passed through a portion of a patient's anatomy. The pattern of X-radiation that passes through the patient is recorded in one or more radiation-sensitive emulsion layers of a radiographic film.
There is no single radiographic element that adequately serves all medical diagnostic needs. The degree to which X-radiation is absorbed varies widely from one anatomical region to the next. For example, lungs, which are filled with air, absorb relatively low levels of X-radiation while much higher levels of X-radiation are absorbed in heart imaging. Also, the feature sought for observation can either differ markedly in its X-radiation absorption from adjacent anatomy, such as a clean break in a bone, or can differ only slightly, such as a lesion or anomaly in soft tissue.
Mammographic diagnostic needs challenge radiographic imaging capabilities. An advanced tumor or cancer can be easily identified, but the diagnostic goal, to maximize survival rates, is to identify cancerous and pre-cancerous growths at the earliest possible stage of development. This is a challenge, since the anatomical feature being sought is, in its earliest stages, a tiny microcalcification and the difference in X-radiation absorption between that feature and healthy tissue is not large.
The types of radiographic elements most generally used for medical diagnostic imaging employ tabular grain emulsions, usually coated on both sides of a transparent film support--i.e., dual coated. Tabular grain emulsions offer many advantages, including the following particularly relevant advantages: increased covering power (allowing reductions in silver coating coverages) and resistance to covering power loss as a function of increased hardening (allowing radiographic elements to be fully forehardened, thereby simplifying processing).
Attempts have been made to apply separately and together tabular grain emulsions and dual coated formats to mammographic imaging applications, both illustrated by Luckey et al U.S. Patent No. 4,710,637. Unfortunately, the performance of radiographic elements containing tabular grain emulsions has not met the high performance requirements necessary for acceptance for mammographic imaging.
Medical diagnostic radiographic elements intended for mammographic imaging that have been most widely accepted by radiologists contain a single radiation-sensitive emulsion layer containing non-tabular silver halide grains. The single sided emulsion coating format maximizes image sharpness as compared to the more generally used dual coated format. The non-tabular silver halide grains allow higher contrasts, particularly higher lower scale contrasts, but, to realize acceptable maximum densities without coating excessive levels of silver, the non-tabular grains require that hardening be completed during processing--i.e., the radiographic elements are only partially forehardened. This results in increased water ingestion into the radiographic element during processing and, as a consequence, limits the extent to which overall processing times can be reduced. Single sided mammographic radiographic elements can be processed in less than 90 seconds, but are incapable of satisfying significantly lower overall processing cycle times. Radiographic element A described as a control in the Examples below is representative of single sided mammographic radiographic elements of the type currently accepted for mammographic medical diagnostic imaging.
The rhodium doping of radiation-sensitive silver halide grains is a known technique for increasing contrast. Keller Science and Technology of Photography, VCH, New York, 1993, at page 40 states:
A fundamentally different approach to high gradation values is the doping of the emulsion grains with heavy-metal ion such as those of rhodium, cadmium, lead and bismuth. Doping pushes back the toe of the characteristic curve and produces a steep gradation. PA1 SS Screen Support PA1 LE Luminescence Emitting Layer PA1 IHC Imaging Hydrophilic Colloid Layer Unit PA1 S Support PA1 AHC Antihalation Hydrophilic Colloid Layer Unit PA1 A fundamentally different approach to high gradation values is the doping of the emulsion grains with heavy-metal ion such as those of rhodium, cadmium, lead and bismuth. Doping pushes back the toe of the characteristic curve and produces a steep gradation. PA1 II. Emulsion Stabilizers, Antifoggants and Antikinking Agents PA1 III. Antistatic Agents/Layers PA1 IV. Overcoat Layers PA1 VII. Antifoggants and stabilizers PA1 IX. Coating physical property modifying addenda
The expression "pushes back the toe" means simply that more light exposure is required before density rises above a minimum level. In other words, increased contrast is obtained at the price of reduced speed.
Increasing mean grain size is a known technique for increasing imaging speed. Unfortunately, granularity (image noise) increases are an inherent consequence of increasing mean grain size. Attempting medical diagnoses with larger grain sizes and therefore grainy images runs a significant risk of failing to identify the presence of microcalcifications.